This invention relates generally to optical communication systems and, more particularly, to modulation and filtering of optical signals.
In optical communication systems, optical signals transport information along optical fibers. Transmitters generate these optical signals by modulating optical carriers using various techniques. In some systems, transmitters use non-return to zero (NRZ) modulation for transmissions. However, at higher bit rates appropriate for long-haul transmissions, NRZ modulated transmissions are highly susceptible to degradations caused by non-linear effects. Thus, for high-bit rate transmissions, systems may use carrier-suppressed, return to zero (CS-RZ) modulation, which has a high tolerance to non-linear effects in optical fibers. However, while CS-RZ modulation achieves a higher tolerance to non-linear effects than NRZ modulation, CS-RZ modulated transmissions suffer from a reduced dispersion tolerance compared to NRZ modulated transmissions.
In accordance with the present invention, techniques for modulation and filtering of optical signals are provided that substantially eliminate or reduce disadvantages or problems associated with previous techniques.
In accordance with one embodiment of the present invention, an optical modulator includes a first modulator that receives an optical carrier signal and modulates the optical carrier signal with a clock signal to generate a clock-modulated optical signal. The optical modulator also includes an optical filter attached to the first modulator. The optical filter suppresses a residual carrier component of the clock-modulated optical signal received from the first modulator. The optical modulator further includes a second modulator attached to the carrier suppression filter. The second modulator modulates the filtered optical signal with a non-return to zero data signal.
Embodiments of the present invention provide various technical advantages. These techniques permit an optical modulator to increase dispersion tolerance of transmitted signals while maintaining a high tolerance to non-linear effects in optical fibers. Thus, the optical modulator is well suited for use in long-haul transmissions of high data rate signals. Moreover, these modulation and filtering techniques permit the use of relatively poor quality components. Therefore, as opposed to previous optical modulators, a modulator according to the present invention may use less expensive components while achieving greater dispersion tolerance. Therefore, these modulators may achieve both increased performance and reduced costs.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages.